Carbon nanotubes and methods of fabrication thereof using a catalyst precursor

ABSTRACT

Carbon nanotubes including single-walled carbon nanotubes (SWNTS) are grown in a manner that facilitates the formation of distinct, individual nanotubes. In one example embodiment of the present invention, SWNT probe-tips for applications such as atomic force microscopy (AFM) are synthesized on silicon pyramids for integration, for example, onto AFM cantilevers. In another implementation, the growth of SWNTs involves dip coating of silicon pyramids with a liquid phase catalyst followed by chemical vapor deposition (CVD) using methane for growing SWNTs. In another implementation, SWNTs are shortened in an inert atmosphere to achieve desirable lengths, for instance, as used in AFM tips. With these approaches, large-scale arrays of nanotubes can be manufactured, for example, using contact printing for catalyst deposition and controllably shortening the nanotubes via an inert discharge.

RELATED PATENT DOCUMENTS

[0001] This is a continuation/divisional of U.S. patent application Ser. No. 09/467,096, filed on Dec. 10, 1999 (STFD.016PA/S99-134), to which Applicant claims priority under 35 U.S.C. §120 for common subject matter.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was supported in part by NSF grant number ECS-9871947, DARPA/ONR grant number 184NO18.

FIELD OF THE INVENTION

[0003] This invention relates generally to carbon nanotubes and, more particularly, to carbon nanotubes and their fabrication using a catalyst precursor.

BACKGROUND

[0004] Carbon nanotubes are unique carbon-based, molecular structures that exhibit interesting and useful electrical properties. There are two general types of carbon nanotubes, referred to as multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs). SWNTs have a cylindrical sheet-like, one-atom-thick shell of hexagonally-arranged carbon atoms, and MWNTs are typically composed of multiple coaxial cylinders of ever-increasing diameter about a common axis. Thus, SWNTs can be considered to be the structure underlying MWNTs and also carbon nanotube ropes, which are uniquely-arranged arrays of SWNTs.

[0005] Due to their unique electrical properties, carbon nanotubes are being studied for development in a variety of applications. These applications include, among others, chemical and bio-type sensing, atomic force microscopy (AFM), field-emission sources, selective-molecule grabbing, nano-electronic devices, and a variety of composite materials with enhanced mechanical and electromechanical properties. For general information regarding carbon nanotubes, and for specific information regarding SWNTs and its applications, reference may be made generally to the above-mentioned patent documents, and also to: “Carbon Nanotubes: Synthesis, Structure, Properties and Applications,” M. S. Dresselhaus, G. Dresselhaus and Ph. Avouris (Eds.), Springer-Verlag Berlin Heidelberg, New York, 2001; and “T. Single-shell Carbon Nanotubes of 1-nm Diameter,” Iijima, S. & Ichihashi, Nature 363, 603-605 (1993).

[0006] Many electronic devices benefit from small-scale electronic circuits and arrangements, and also play an important role in a variety of applications. The size and electrical properties of nanotubes including carbon nanotubes make them potentially useful for such small-scale devices.

[0007] As discussed above, atomic force microscopy (AFM) is one particular application for which carbon nanotubes are useful. AFM has been a powerful tool for a wide range of fundamental research and technological applications. Atomic force microscopes employ a probe tip used to obtain an image of a specimen, with the size and shape of the probe tip being related to the lateral resolution and fidelity of images obtained using the AFM arrangement. Carbon nanotube tips present ideal characteristics for enhancing the capabilities of AFM in imaging, manipulation and nanofabrication due to their sharpness, high aspect ratios, high mechanical stiffness and resilience, and chemical characteristics. AFM tips employing carbon nanotubes exhibit advantages over conventional AFM tips, such as longer durability, deep structure probing capability, and high lateral resolution in imaging and lithographic applications.

[0008] In view of the above, carbon nanotubes exhibit characteristics that make them useful for a variety of implementations. However, such nanotubes have been difficult to manufacture and implement in a variety of such applications. For instance, obtaining individual, high quality, single-walled nanotubes has proven to be a difficult task. Existing methods for the production of nanotubes, including arc-discharge and laser ablation techniques, yield bulk materials with tangled nanotubes. The nanotubes in the bulk materials are mostly in bundled forms. These tangled nanotubes are extremely difficult to purify, isolate, manipulate, shorten and use as discrete elements for making functional devices. Furthermore, many previously-available nanotubes have exhibited molecular-level structural defects, which can result in relatively weak nanotubes with poor electrical characteristics.

SUMMARY

[0009] The present invention is directed to overcoming the above-mentioned challenges and others related to the types of devices and applications discussed above and in other implementations. The present invention is exemplified in a number of implementations and applications, some of which are summarized below.

[0010] According to one example embodiment of the present invention, carbon nanotubes including SWNTs are grown on a support structure, such as a silicon tip and/or a pyramid or cone shaped tip on top of a tower structure. In one particular implementation, the SWNTs are formed having radii less than about 1 nanometer and are implemented for use in AFM. In another implementation, arrays of the SWNTs are formed. In still another implementation, nanostructures having a feature size less than about 10 nanometers are fabricated using the SWNTs.

[0011] In another example embodiment of the present invention, carbon nanotubes are fabricated using a catalyst precursor material, such as a liquid phase catalyst material. A support structure is formed and a portion thereof is coated with the catalyst precursor material. In one implementation, the catalyst precursor material comprises a metal-containing salt and a long-chain molecular compound dissolved in a solvent. The catalyst precursor material is calcined to form a catalyst material, and the support structure with the catalyst is then exposed to a carbon containing gas while heated, for example, in a tube furnace. One or more nanotubes are grown from the support structure as carbon from the carbon-containing gas is deposited thereon using the catalyst material to catalyze a reaction of the carbon-containing gas.

[0012] In another example embodiment of the present invention, an array of carbon nanotube probe tips are grown on a substrate having an array of support structures with a catalyst precursor material thereon. In one implementation, a stamp is coated with a liquid phase precursor material and contacted to the support structure array to transfer the catalyst precursor material thereto. The array is then exposed to a carbon containing gas and heated to form carbon nanotubes on the support structures, such as discussed above. In one implementation, the catalyst precursor material is applied to the support structures without coating portions of the substrate between the support structures.

[0013] In still another example embodiment of the present invention, carbon nanotubes are shortened in a controllable fashion using an inert discharge. A voltage is applied to the carbon nanotubes in an atmosphere having an inert gas. The inert discharge may, for example, be used to shorten nanotubes grown in connection with one or more of the example embodiments and implementations thereof discussed above. In one implementation, a nanotube is shortened via inert discharge shortening by contacting the nanotube to a heavily-doped silicon substrate and applying a voltage between the nanotube and the substrate.

[0014] The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and detailed description that follow more particularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention may be more completely understood in consideration of the detailed description of various embodiments of the invention that follows in connection with the accompanying drawings, in which:

[0016] FIGS.1A-1C show the growth of oriented SWNTs on pyramidal AFM tips in connection with one or more example embodiments of the present invention, in which:

[0017]FIG. 1A shows a tip structure having a catalyst being formed thereon; and

[0018]FIGS. 1B and 1C show images of carbon nanotubes grown from a tip structure similar to that shown in FIG. 1A;

[0019] FIGS. 2A-2D show the growth of single-walled carbon nanotubes on a large-scale support structure array, according to another example embodiment of the present invention, in which:

[0020]FIG. 2A shows a substrate having tip structures and a stamp having a catalyst precursor thereon;

[0021]FIG. 2B shows the stamp being applied to the tip structures of FIG. 2A;

[0022]FIG. 2C shows the tip structures of FIG. 2B having a catalyst precursor material formed thereon; and

[0023]FIG. 2D shows carbon nanotubes grown from the tip structures of FIG. 2C;

[0024]FIG. 3 shows a support structure for growing a nanotube tip, according to another example embodiment of the present invention;

[0025]FIG. 4A shows a SEM image of a silicon tip-on-tower array, according to another example embodiment of the present invention;

[0026]FIG. 4B shows a schematic of a catalyst transfer process involving contact-printing, according to another example embodiment of the present invention;

[0027]FIG. 4C shows SEM images of SWNTs grown off silicon tips in accordance with one or more of the example embodiments discussed herein;

[0028] FIGS. 5A-5B show force plots of a SWNT before and after inert discharge shortening thereof, according to another example embodiment of the present invention;

[0029]FIG. 6A shows an AFM image of λ-DNA molecules on a mica surface recorded by an individual SWNT probe tip fabricated in connection with an example embodiment of the present invention; and

[0030]FIG. 6B shows TiO₂ lines (bright structures) fabricated on a Titanium thin film using a SWNT probe tip in connection with another example embodiment of the present invention.

[0031] While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

DETAILED DESCRIPTION

[0032] The present invention is believed to be applicable to a variety of different types of devices, and the invention has been found to be particularly suited for nanotube devices, such as AFM probe tips, and the growth of nanotubes using a catalyst material disposed on a tip-type structure. While the present invention is not necessarily limited to such applications, various aspects of the invention may be appreciated through a discussion of various examples using this context.

[0033] According to an example embodiment of the present invention, carbon nanotubes are grown using methane CVD synthesis of SWNTs using a liquid-phase catalyst precursor material. In one implementation, the liquid-phase precursor material includes three components: a metal containing salt, a long-chain molecular compound, and a solvent. The salt may, for example, include a chloride, sulfate or nitrate material. The long-chain molecule may, for example, include a surfactant, such as soap, or a polymer. The solvent may include one or more of a variety of organic or inorganic solvents, e.g., an alcohol, acetone or water. The liquid-phase catalyst precursor is deposited on a structure and used to form a catalyst for reacting the methane for CVD growth of the SWNTs.

[0034] In a specific example implementation, the salt is a mixture of inorganic chlorides, the long-chain molecular compound is a polymer that serves as a structure-directing agent for the chlorides and the solvent is an alcohol, such as ethanol, methanol or butanol. Exemplary block copolymers that may be used in connection with this example implementation include tri-block copolymers such as pluronic P-123 poly(alkylene oxide) HO(CH₂CH₂O)₂₀—(CH₂CH(CH₃)O)₇₀—(CH₂CH₂O)₂₀OH (available from BASF, Inc., of Mount Olive, N.J.). Exemplary inorganic chlorides that may be used in connection with this example implementation include AlCl₃, SiCl₄, FeCl₃ and MoO₂Cl₂. In addition, sulfates, nitrates or other types of salt may be used in place of or in addition to the organic chloride.

[0035] In one implementation, aluminum and silicon chlorides are processed via hydrolyses, gellation and polymerization into a network of oxides with the block-copolymer phase directing the structure of a network. Small amounts of iron and molybdenum chlorides, relative to the aluminum and silicon chlorides, are added to the precursor mixture, which leads to a material containing finely dispersed metal oxide catalytic nanoparticles on an alumina/silica matrix. The catalytic nanoparticles serve as active sites for SWNT synthesis. The liquid phase catalyst precursor is formed in one or more thin layers, for example, through dip-coating, spin-coating, patterning transferring, or contact printing techniques. For information regarding a general approach of using block copolymers and inorganic chlorides to produce high surface area mesoporous materials, and for specific information regarding such applications that may be used in connection with one or more example embodiments of the present invention, reference may be made to Yang et al, Nature, 1998, 396, 152 and to Yang et al, Science, 1998, 282, 2244, both of which are fully incorporated herein by reference.

[0036] In another implementation, the catalyst precursor material is formed by first dissolving AlCl₃.6H₂O (about 1.2 g) in methanol (about 9 mL) followed by the addition of SiCl₄ (about 0.6 mL) to form a first solution. A second solution is prepared by dissolving FeCl₃.6H₂O (about 0.09 g) and MoCl₂ 0 ₂ (about 0.006 g, mmol) in ethanol (about 2 mL). A third solution of P-123 (about 1.0 g) in ethanol (about 5 mL) is then combined with the first and second solutions and stirred for about 24 hours. The mixture is aged at room temperature for about 1-3 days to form the catalyst precursor. Solvent is removed from the catalyst precursor, followed by calcination of the catalyst precursor in air to form a precursor material containing iron-oxide nanoparticles supported on a mixed alumina/silica oxide. Such a material is active for the synthesis of SWNTs by chemical vapor deposition (CVD). Although the above example recites specific quantities of chlorides and solvents, a variety of quantities of precursor material may be formed using appropriate (similar) ratios of the above materials based on the above quantities. For instance, the ratio of FeCl₃ to ethanol can be increased or decreased by a factor of 10 with respect to the ratio described above, while achieving a catalyst precursor suitable for the growth of carbon nanotubes.

[0037] Referring to FIG. 1A, a liquid phase catalyst precursor 105 is formed on a tip 100 disposed on a structure 110, and subsequently calcined to form a catalyst material 108, according to another example embodiment of the present invention. The catalyst material 108 is then used to grow a carbon nanotube from the tip 100, with example nanotubes grown using this approach shown in scanning electron microscope (SEM) and transmission electron microscope (TEM) images respectively in FIGS. 1B and 1C, which can be grown substantially free of amorphous carbon. The tip 100 may, for example, include a silicon pyramidal tip integrated onto commercially available AFM cantilevers (e.g., structure 100). Cantilevers that may be used in connection with the present invention include the FESP type cantilever having a spring constant of about 1N/m (Newton/meter) and manufactured by Digital Instruments of Santa Barbara, Calif. The support structure 110 and tip 100 may be formed by any suitable technique such as microlithography, ion milling or etching.

[0038] In one implementation, under an optical microscope equipped with mechanical manipulators, a silicon pyramid tip 100 is dipped into a precursor solution (e.g., contained in a micro-pipette) and pulled out, leaving a drop of liquid catalyst precursor 105 on the silicon pyramid tip 100. Due to high surface tension at the tip 100, the droplet tends to retract from the tip of the pyramid and settle around the base of the pyramid, as shown. This dip-coating process may be repeated multiple times (e.g., 5-10) until the catalyst precursor 105 in a gel-like state covers approximately half of the pyramid height as shown in FIG. 1A. The tip 100 is then calcined in air at 400° C. for about 1 hour and at 700° C. for about ½ hour evaporate solvent and remove a polymer component from the catalyst precursor to form a thin layer of catalyst 108 surrounding the base of the tip 100 and partially coating the sides thereof. The calcining may include, for example, heating a solid or gel-state material to a temperature below its melting point to create a condition of thermal decomposition.

[0039] The nanotubes 120 and 130 shown in FIGS. 1B and 1C are grown from tips 122 and 132, respectively, by forming a catalyst thereon as discussed in connection with FIG. 1A and subjecting the tips to chemical vapor deposition using a carbon-containing gas. In one implementation, the carbon containing gas includes methane and the tips 122 and 132 are heated to approximately 900° C. for about 15 minutes in a 1-inch tube furnace under a methane flow rate of about 1000 mL/min. In another implementation, a carbon containing gas that does not pyrolize at temperatures between about 800° C. and about 1000° C. is used in addition to or in replacement of the methane. In addition, the carbon containing gas may include small concentrations of hydrocarbons such as ethane, butane, propane or benzene.

[0040] In connection with another example embodiment of the present invention, it has been discovered that a conditioning step of the CVD growth chamber significantly increases the yield of SWNTs grown on pyramidal silicon tips, such as discussed in connection with FIGS. 1A-1C above. In one implementation, the conditioning involves suspending a supported catalyst material in methanol and coating an inner wall of a 1 inch quartz tube reactor with a layer of the catalyst. In connection with this implementation, it has been discovered that SWNT tips grow at a success rate of approximately 20% using a non-conditioned quartz tube reactor, whereas the conditioning step increases the success rate of growing SWNTs on pyramids to approximately 90%. In another implementation, reactive hydrocarbon species (e.g., higher order hydrocarbons and/or radicals such as CH_(n) or C_(m)H_(n)) are generated by the reactions between CH₄ and the catalyst in the chamber environment. These species feed the growth of SWNTs on the pyramids more effectively than CH₄ alone.

[0041] In another implementation, during CVD growth of a nanotube from the pyramid tip 100 in FIG. 1A, SWNTs nucleate near the base portion of the tip and lengthen in various directions. As growth terminates, the nanotube (or bundle of nanotubes) adheres to the surface of the pyramid tip 100 and extends off the tip, thereby maximizing tube-surface van der Waals interactions. The overlapping section between the pyramid tip 100 and the SWNT can be as great as about 10 micrometers in length, or the height of the pyramid tip. A substantial number of SWNTs may also be grown in the plane of the structure 105 and situated away from the tip of the pyramid such that they generally do not interfere with an extended SWNT (e.g., nanotube 120 and/or 130), which is useful, for example, when using the SWNT as a probe.

[0042] FIGS. 2A-2D shows various stages of manufacture of an array of carbon nanotubes, according to another example embodiment of the present invention. The array may be grown using, for example, one or more of the nanotube growth approaches discussed above, using a liquid phase precursor material to facilitate the production of the array. In FIG. 2A, an array 210 of a plurality of support structures including structure 220 is shown formed on a substrate 230. The support structure 220 may, for example, include pyramidal or conical silicon tip arrays obtained by microfabrication processes such as photolithography, dry etching and oxidation-sharpening. A layer of catalyst precursor material 240 is coated onto a flat stamp 255. In one implementation, stamp 250 is made from an elastic material such as polydimethylsilane (PDMS). Stamp 250 is coated with precursor material 240 by any suitable means. In one implementation, the stamp 250 is coated via spin coating, wherein the stamp 240 spins at a rate of about 5000 revolutions per minute (rpm) for about 10 seconds.

[0043] In FIG. 2B, the coated stamp 250 is pressed against substrate 230 to transfer the catalyst precursor material 240 to the array 210 of support structures. The coated support structure array 210 is then calcined as shown in FIG. 2C, for example, in a manner consistent with the examples discussed above in connection with FIG. 1A. Using support structure 220 as an example, the calcining results in calcined catalyst material 240 extending over at least a portion of the support structure 220. In FIG. 2D, nanotubes including nanotubes 260 and 261 are grown on the array 210 of support structures using, for example, the CVD approach discussed above.

[0044]FIG. 3 shows support structures 310 and 311 in a tower-tip form and extending from a substrate 316, according to another example embodiment of the present invention. The support structures 310 and 311 may be used, for example, in connection with the example embodiments discussed above with FIGS. 2A-2D (e.g., as a replacement for or in addition to the array 210 of support structures). The structures 310 and 311 include cone-shaped tips 312 and 313 disposed on top of towers 314 and 315, respectively. This approach is particularly conducive to deposition of catalyst precursor material onto sides of the cone-shaped tips 312 and 313, for example, using a stamping approach similar to that discussed above. This tower-tip approach also inhibits catalyst material from collecting on portions 318 of the substrate 316 between support structures 310 and 311 during stamping.

[0045]FIG. 4A shows an SEM image of columns 406 and 408 of an array of silicon tip-on-tower support structures that may, for example, be formed in connection with one or more of the example embodiments discussed above. The height of the cone-shaped tips (e.g., tip 402) is about 16 micrometers and the height of the towers (e.g. 404) is 55 micrometers.

[0046]FIG. 4B shows tip-on-tower structures 412 and 413, such as those shown in FIG. 4A, undergoing a catalyst transfer process by contact-printing in a manner not inconsistent with the example embodiments discussed in connection with FIGS. 2A-2D. The printing is effected using a pad 450 having a layer of catalyst material 440, which is transferred to the tip-on-tower structures 412 and 413 for subsequent growth of nanotubes therefrom.

[0047]FIG. 4C shows SEM images of SWNTs grown off silicon tips 472, 473 and 474, such as those shown in FIG. 4A, with the SWNTs oriented about normal to a substrate, according to another example embodiment of the present invention. In one implementation, the silicon tip-on-tower structures 472, 473 and 474 (or, e.g., similar structures formed in an array as shown in FIG. 4A) are arranged on an array of AFM cantilevers fabricated on a silicon wafer. With this approach, CVD synthesis can be combined with the carbon nanotube growth approaches discussed herein to form molecular probe tips in a large-scale approach.

[0048] In another example embodiment of the present invention, carbon nanotubes are shortened using an inert discharge shortening approach. In many applications, including those discussed above, SWNTs extend between 1-20 micrometers in length beyond growth structures, such as the pyramid tip 100. It is sometimes desirable to shorten the nanotubes to about 30-100 nanometers in length, such as for forming rigid AFM probe tips (e.g., as shown in FIG. 1C). In order to shorten the nanotubes, a voltage is applied thereto in the presence of inert gas, which results in an electrical arc discharge. Inert atmospheres that can be implemented in connection with this example embodiment include nitrogen (N₂) and noble gases such as helium, neon, argon, xenon and krypton. In one implementation, SWNTs are shortened in Argon using between about 20-50 volts applied between the SWNT and a substrate. In such an inert atmosphere, the concentrations of oxygen and other reactive molecules are relatively low compared to ambient conditions, which significantly limits the probability and length scale of a discharge event near the end of a sharp tip under high electric fields. With this approach, the length of nanotubes including SWNTs can be controlled.

[0049]FIGS. 5A and 5B show force calibration curves before and after, respectively, inert discharge shortening of a relatively long (about 5 micrometers prior to shortening) SWNT on a cantilever tip, according to another example embodiment of the present invention. In FIG. 5A, cantilever amplitude is plotted vs. distance for an as-grown carbon nanotube. The cantilever is oscillated at the amplitude shown, with the as-grown nanotube contacting, or tapping, across a surface. During extension (solid curve 510), the SWNT is too soft to cause a noticeable decrease in the cantilever amplitude as it crashes into a substrate (e.g., the nanotube bends rather than causing a decrease in the amplitude). During retraction (dashed curve 520), however, the amplitude response curve shows oscillating variations, indicating ‘stick-slip’ motions of the nanotube as it is pulled off the substrate (e.g., the nanotube stick and slips at atom sites or unit cells on the substrate). In the amplitude vs. distance curve of FIG. 5B, the nanotube has been shortened to less than about 40 nanometers, resulting in a stiff SWNT that can be used as a probe tip. The cantilever tip is again oscillated, with the amplitude recovery beyond full tube-substrate contact being due to buckling of the SWNT. Little hysteresis exists between extension 530 and retraction 540 curves, which is a desired characteristic of high quality probe tips for tapping mode AFM.

[0050] In another example embodiment of the present invention, a stream of Argon is directed over a cantilever mounted in an AFM with a SWNT extending from a structure on the cantilever. The SWNT is brought into contact with a heavily doped silicon substrate and monitored by a cantilever amplitude vs. distance curve, such as that discussed above in connection with FIGS. 5A and 5B. A voltage is then applied between the SWNT and the substrate and gradually increased until the loss of nanotube-substrate contact occurs as a result of nanotube shortening. This approach is particularly useful, for example, for reducing the length of SWNTs in steps of about 30 nanometers, providing an excellent control of the length of SWNT probes.

[0051] In another example embodiment of the present invention, SWNTs are aligned to a substrate using an inert discharge shortening approach, such as discussed above. The shortening process results in strong electrostatic forces between an SWNT and a substrate to which the nanotube is brought into contact, which has also been discovered to promote normal alignment of the SWNT to the substrate. The orientation of the substrate, relative to the SWNT, is selected to achieve a desired SWNT orientation. For instance, when shortening SWNTs extending from a cantilever, it is often desirable for the SWNT to be perpendicular to the cantilever. In this instance, the substrate is oriented parallel with the cantilever, such that the SWNT is oriented perpendicular to the cantilever. With this approach, the inert discharge shortening process helps to orient the SWNT for use in a variety of applications.

Experimental Results

[0052] In connection with one or more of the various example embodiments and implementations discussed above, a variety of experimental results were obtained, with summaries below. These experimental results include AFM imaging of double-stranded λ-DNA (from Life Technologies, Gaithersburg, Md.) absorbed onto freshly cleaved mica surfaces. FIG. 6A shows the structure of linear DNA molecules probed by an individual SWNT in air. The imaging was conducted using SWNT AFM tips fabricated as described above. The SWNT radius was approximately 1 nanometer. The inset 610 of FIG. 6A shows the topography along a line cut across a DNA molecule. The full-width-at-half-maximum (FWHM) of the DNA molecule is measured to be 3 nm along the molecule, closely approaching the true width (about 2 nanometers) of double-stranded DNA. The DNA strands also exhibit fine structures along the length, with quasi-periodic corrugations that are spaced at approximately 3-4 nanometer distance, close to the about 3.4 nanometer helical pitch.

[0053] Imaging in aqueous solutions with individual SWNT probe tips should allow high order DNA structures to be clearly resolved with the molecules in native environments and make small probing forces possible due to the absence of undesired capillary forces. Systematic imaging in air with the SWNT tips described above consistently gave apparent widths in the range of 3-5 nanometers for λ-DNA. In comparison, the apparent widths of DNA molecules were about 15±5 nanometers observed by using conventional pyramidal tips, and were about 10 nanometers by multi-walled nanotube probes. These results show that individual SWNT tips are promising in improving the lateral resolution of AFM imaging of biological systems to the submolecular level.

[0054] Scanning probe lithographic fabrication of oxide nanostructures on metal substrates was also carried out with SWNT tips synthesized in connection with one or more of the examples discussed herein. Miniaturization by existing microfabrication methods has previously been limited to the sub-micron scale. Scanning probe lithography may provide a viable route to future nanoscale devices with high throughput in imaging and nanofabrication achievable through AFM equipped with parallel probe arrays. Previous scanning tunneling microscopy work fabricated structures with near atomic resolution under ultra-high vacuum conditions. However, feature sizes obtainable with AFM operating under ambient conditions have been less than about 10 nanometers, limited by the size of the probe tips.

[0055] The SWNT tips fabricated as described above were able to fabricate TiO₂ nanostructures with feature size below about 10 nanometers. The substrate used was a smooth Titanium film deposited onto atomically flat single crystal α-Al₂ 0 ₃. TiO₂ lines (bright structures) 6 nm wide (FWHM) were fabricated on a Titanium thin film deposited onto an α-Al₂ 0 ₃ substrate. The apparent height of the TiO₂ is about 0.8 nanometers. The oxide structures were obtained by applying bias voltages modulated between a−8.5 volt (tip relative to substrate) and +0.5 volt during tapping mode scanning at a rate of about 120-160 micrometers/s. The average tip-substrate distance was maintained at approximately 1 nanometers during the lithographic scan.

[0056] An AFM image recorded by the same SWNT tip after lithographic writing is shown in FIG. 6B. The FWHM of TiO₂ lines is about 6 nanometers. Using SWNT tips, about 6 nanometer TiO₂ nanodots spaced at about a 10 nanometer pitch with a packing density of about 1 Tera-bit per square inch (data not shown) was also fabricated. SWNT AFM tips thus enable the fabrication of nanostructures with feature sizes below the previous 10-nanometer barrier for AFM operation under ambient conditions. This advancement is useful in electronic, recording and other types of miniaturized devices.

[0057] The various embodiments described above are provided by way of illustration only and should not be construed to limit the invention. Based on the above discussion and illustrations, those skilled in the art will readily recognize that various modifications and changes may be made to the present invention without strictly following the exemplary embodiments and applications illustrated and described herein. For instance, such changes may include modifying the nanotubes for one or more particular applications, altering the nanotube arrangements, and where appropriate, using SWNTs as building blocks for more complex devices. Moreover, the nanotubes may be made of materials other than carbon, such as silicon and boron, which can also be grown using a synthesis process similar to that described above. Such modifications and changes do not depart from the true spirit and scope of the present invention, which is set forth in the following claims. 

What is claimed is:
 1. A carbon nanotube probe tip comprising: a support structure comprising a tip protruding from a substrate; and a carbon nanotube extending from the tip.
 2. The carbon nanotube probe tip of claim 1, wherein the tip is in the shape of a pyramid.
 3. The carbon nanotube probe tip of claim 1, wherein the tip is in the shape of a cone.
 4. The carbon nanotube probe tip of claim 1, wherein the support structure includes a wider base portion and a narrower tip portion above the base portion and wherein the carbon nanotube extends from the lower base portion, along the upper tip portion and extending therefrom.
 5. The carbon nanotube probe tip of claim 4, wherein the lower base portion has a catalyst material disposed thereon, the catalyst material being configured and arranged for growing the carbon nanotube.
 6. The carbon nanotube probe tip of claim 1, wherein the support structure has a catalyst material disposed thereon, the catalyst material being configured and arranged for growing the carbon nanotube.
 7. The carbon nanotube probe tip of claim 6, wherein the catalyst material comprises a metal-containing salt, a long-chain molecular compound and a solvent.
 8. The carbon nanotube probe tip of claim 7, wherein the catalyst material includes a calcined material that includes metal oxide particles and at least one of: alumina and silica.
 9. The carbon nanotube probe tip of claim 1, wherein the carbon nanotube is a single-walled carbon nanotube.
 10. The carbon nanotube probe tip of claim 1, wherein the support structure includes a cantilever and wherein the tip protrudes from the cantilever.
 11. The carbon nanotube probe tip of claim 1, wherein the support structure includes a tower and wherein the tip is disposed on the tower.
 12. A liquid phase precursor for fabricating one or more carbon nanotubes comprising: a metal-containing salt; a long-chain molecular compound; and a solvent.
 13. The precursor of claim 12, wherein the metal-containing salt is selected from the group of: chloride, sulfate and nitrate.
 14. The precursor of claim 13, wherein the chloride is an inorganic chloride.
 15. The precursor of claim 14, wherein the inorganic chloride includes AlCl₃, SiCl₄, FeCl₃ and MoO₂Cl₂.
 16. The precursor of claim 15, wherein the long chain molecule is a surfactant or a polymer.
 17. The precursor of claim 16, wherein the long chain molecule is a tri-block copolymer comprising pluronic P-123 poly(alkylene oxide) HO(CH₂CH₂O)₂₀—(CH₂CH(CH₃)O)₇₀—(CH₂CH₂O)₂₀OH.
 18. The precursor of claim 12, wherein the solvent is selected from the group of: alcohol, acetone and water.
 19. The precursor of claim 12, wherein the metal-containing salt, the long-chain molecular compound and the solvent are configured and arranged to form a catalyst material for growing a carbon nanotube in the presence of a carbon-containing gas.
 20. The precursor of claim 19, wherein the metal-containing salt, the long-chain molecular compound and the solvent are configured and arranged to form a catalyst material when calcined.
 21. A method for manufacturing a single-walled nanotube (SWNT), the method comprising: introducing an inert gas to the SWNT; and applying a voltage between the SWNT and a substrate and shortening the SWNT.
 22. The method of claim 21, further comprising contacting the SWNT to the substrate.
 23. The method of claim 22, wherein the substrate is a heavily-doped silicon substrate.
 24. The method of claim 22, wherein applying a voltage between the SWNT and the substrate includes gradually increasing the voltage applied between the SWNT and the substrate until a loss of nanotube-substrate contact occurs.
 25. The method of claim 21, further comprising: using a force calibration curve to detect a stiffness characteristic of the SWNT; and in response to detecting a stiffness characteristic that indicates that the SWNT is not sufficiently stiff, repeating the steps of introducing an inert gas to the SWNT and applying a voltage between the SWNT and a substrate and shortening the SWNT.
 26. The method of claim 21, wherein the SWNT is disposed on an AFM cantilever and wherein applying a voltage between the SWNT and a substrate includes manipulating the cantilever to contact the SWNT to the substrate.
 27. The method of claim 21, wherein applying a voltage between the SWNT and a substrate includes using the voltage to align the SWNT in a direction generally normal to the substrate. 